1. Field of the Invention
The present invention relates to an apparatus and method for measuring the level of a liquid in a tank or container. More specifically, the present invention relates to the use of an ultrasonic or acoustic transducer for sensing the level of the liquid within the container.
2. Description of the Prior Art
It is often necessary to measure the level of a liquid in a tank or reservoir under conditions which preclude normal measurement techniques. For example, the liquid may be stored under conditions of high temperature or humidity, or the liquid itself may be highly combustible or may emit fumes or odors. Also, the configuration of the container may be such that it is not practical to place the sensor in a position where it is generally perpendicular to the level of the liquid, and thus a somewhat indirect method of measuring the level of the liquid must be used.
Rosie et al in U.S. Pat. No. 4,229,798 discloses the use of an ultrasonic transducer for measuring the vertical height of a liquid in a tank. His invention also includes means for compensating for the change in velocity of propagation of the ultrasonic waves within the tank as the temperature of the liquid and the vapors above the liquid change. A microprocessor based system is utilized for calculating the volume of the liquid within the tank based upon measurement of the distance from the top of the tank to the fluid, and thereafter adjusting for errors induced by temperature, etc. However, the inventor recognizes that the use of a capacitive sensor would be preferable to an ultrasonic sensor when the tank containing the liquid is tilted (e.g., in mobile tankers) such that the beam from the ultrasonic transducer may not arrive perpendicular to the surface of the liquid.
Austin et al in U.S. Pat. No. 4,170,765 utilizes an ultrasonic pulse transducer which is mounted within one end of a pipe that is then extended into a fluid such that the ultrasonic pulses propagate along the cavity of the tube in a direction generally perpendicular to the surface of the liquid. A reflection target, such as a discontinuity in the tube above the liquid surface, is used to provide an echo (or calibration signal) which arrives back at the ultrasonic sensor before the reflection from the surface of the liquid. In this manner the calibration signal can be analyzed for changes induced by environmental conditions such as temperature, humidity and pressure of the gaseous medium above the liquid.
Massa in U.S. Pat. No. 4,210,969 utilizes a small disk target which is suspended within the tank above the level of the liquid. The disk is illuminated by an ultrasonic transducer so as to provide a self-calibrating signal used by the computer for automatically correcting for errors in measurement of the distance from the transducer to the surface of the liquid, such as those which would occur as a result of variations of the velocity of sound in the space above the liquid.
It should be noted that the aforementioned references are designed for use in stationary storage tanks, such as water or fuel reservoirs, that are not subject to movement when measurements are being taken. Neither of the references identify or address the problem of using ultrasonic waveforms directed toward the surface of the liquid from a direction other than perpendicular, or when the tank is moving and subject to surface irregularities (such as standing surface waves or the like).